Refactor t-address encoding
Includes code cherry-picked from the following upstream PRs:
- bitcoin/bitcoin#11117
- bitcoin/bitcoin#11259
- Only the second commit (first is for QT code)
- bitcoin/bitcoin#11167
- Only the first commit (the rest are not part of the t-address encoding refactor).
Part of #3058. Precursor to #3202.
We do not need to be able to calculate multiple SignatureHash versions for a
single transaction format; instead, we use the transaction format to determine
the SigVersion.
The consensus branch ID *does* need to be passed in from the outside, as only
the caller knows the context in which the SignatureHash is being calculated
(ie. mempool acceptance vs. block validation).
JoinSplit signature verification has been moved into ContextualCheckTransaction,
where the consensus branch ID can be obtained.
The argument to the sign command for zcash-tx has been modified to take a height
in addition to the optional sigtype flags.
Simple implementation does not try to optimize coin or note selection.
Caller can send from a taddr or zaddr to multiple recipients.
Currently only one of the recipients can be a zaddr.